Method and apparatus for data compression in memory devices

ABSTRACT

A test circuit for a memory device having a pair of arrays each of which includes a plurality of memory cells arranged in rows and columns. A pair of complementary digit lines is provided for each column of each array. The digit lines are selectively coupled to a pair of I/O lines for each array which are, in turn, coupled to a pair of complementary data lines. The data lines are coupled to respective inputs of a DC sense amplifier, one of which is provided for each array. A multiplexer connects the pair of I/O lines for either one of the arrays to the data lines in a normal operating mode. Thus, in the normal operating mode, data are selectively coupled to the inputs of the DC sense amplifier from the complementary digit lines for an addressed column. In a test mode, the multiplexer connects the I/O lines for both arrays to the data lines to compress the data from the two arrays. Combinatorial logic then determines if both of the data lines have the same logical value, indicating disagreement between the data from the memory arrays that may indicate the presence of a defective memory cell in one or the other array. Thus, in the test mode, data are simultaneously coupled to the inputs of the DC sense amplifier from respective digit lines coupled to two different memory cells, thereby increasing the rate at which background data that has been written to the arrays can be read from the arrays.

TECHNICAL FIELD

This invention relates to integrated circuit memory devices, and, moreparticularly, to a method and apparatus for reading data from memorydevices in a compressed manner to expedite testing of memory devices.

BACKGROUND OF THE INVENTION

Integrated circuits are extensively tested both during and afterproduction and, in some cases, routinely during use after they have beeninstalled in products. For example, memory devices, such as dynamicrandom access memories (“DRAMs”), are tested during production at thewafer level and after packaging, and they are also routinely tested eachtime a computer system using the DRAMs executes a power up routine whenpower is initially applied to the computer system. DRAMs are generallytested by writing known data to each location in the memory, readingdata from each memory location and then determining if the read datamatches the written data. As the capacity of DRAMs and other memorydevices continues to increase, the time required to write and then readdata from all memory locations continues to increase, even though memoryaccess times continue to decrease.

Various proposals have been made to decrease the time required to testmemory devices, such as DRAMs. The time required to write known data tomemory devices has been reduced by such approaches as simultaneouslywriting the same data to each column of each array in the memory deviceone row at a time. Other approaches include internal circuitry fortransferring data from each column of one row to the next withoutrequiring the memory to be addressed. These approaches have reduced thetime required to write known data or a known pattern of data to thememory array.

Solutions have also been proposed for reducing the time required to readdata from memory devices so that the data can be compared to the datawritten to the memory devices. One approach to solving the problem ofexcessive read times relies on data compression in which data from adirect current sense amplifier for one memory array are internallycompared to the data from a sense amplifier of a different memory array.Simultaneously reading data from different memory locations in twodifferent arrays has significantly reduced the time required to readdata from memory devices for test purposes. However, combining data frommultiple sense amplifiers requires the addition of circuitry andconductors to couple the outputs of the multiple sense amplifiers tocomparison circuitry. As the circuit features of semiconductor devicesbecome more densely packed, there is less space available to routeconductors from each sense amplifier to a single location and to provideadditional circuitry to combine the outputs of the direct current senseamplifiers.

There is therefore a need to be able to read data from a memory devicein a compressed form to reduce testing time that can be implemented onintegrated circuits having very densely packed circuit features.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a test circuitis included in a memory device having at least one array or bank ofmemory cells arranged in rows and columns, a pair of complementary digitlines for each column and a direct current sense amplifier that couplesdata from a digit line selected by a column address to an external dataterminal of the memory device. In accordance with one aspect of theinvention, the test circuit includes a direct current sense amplifierthat also provides a wired-OR function, combining data from severaldigit lines to one set of complementary I/O lines. When data from onememory cell differs from data from another cell and these data are beingcombined, both of the complementary I/O lines are low, and a detectorcircuit produces a “fail” signal. As a result, data can be read from thearray in compressed fashion, e.g., from two columns at a time, therebyincreasing the rate at which the memory array can be tested afterbackground data have been written to the array. Although the memorydevice may have only a single memory array, it may also have multiplearrays, in which case columns from different arrays may be coupled tothe inputs of a DC sense amplifier in the test mode. The couplingbetween the inputs of the DC sense amplifier and the digit lines ispreferably through a pair of data lines coupled to the inputs of the DCsense amplifier, a pair of I/O lines for each of the arrays to which thedigit lines of an addressed column are connected and a multiplexerselectively coupling two of the I/O lines to the data lines.

In accordance with an aspect of the invention, the test circuit may beused in a memory device that is connected to a test system. In suchcase, the test system may first write a known pattern of background datato the array followed by reading data from pairs of different columns ofthe memory device, as explained above. The read data are then examinedto determine if the read data correspond to the pattern of backgrounddata written to the array, thereby providing an indication of whetherthe memory device is operating properly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a conventional memory devicethat may use an embodiment of a test circuit in accordance with thepresent invention.

FIG. 2 is a simplified schematic diagram of a prior art DC senseamplifier than can be used with an embodiment of the present invention.

FIG. 3 is a simplified schematic diagram of a portion of a memory devicein accordance with an embodiment of the present invention.

FIG. 4 is a block diagram of a portion of the memory device of FIG. 1including a test circuit in accordance with an embodiment of the presentinvention.

FIG. 5 is a block diagram of the memory device of FIGS. 1 and 4 usedwith a test system to determine whether the memory device is operatingproperly in accordance with embodiments of the present invention.

FIG. 6 is a block diagram of a computer system using the memory deviceof FIGS. 1 and 4 in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a memory device that can advantageously use anembodiment of a test circuit in accordance with the present invention isillustrated in FIG. 1. The memory device shown in FIG. 1 is asynchronous dynamic random access memory (“SDRAM”) 10, although the testcircuit may also be used in other DRAMs and other memory devices. TheSDRAM 10 includes an address register 12 that receives either a rowaddress or a column address on an address bus 14. The address bus 14 isgenerally coupled to a memory controller (not shown in FIG. 1).Typically, a row address is initially received by the address register12 and applied to a row address multiplexer 18. The row addressmultiplexer 18 couples the row address to a number of componentsassociated with either of two memory banks 20, 22, depending upon thestate of a bank address bit forming part of the row address. Associatedwith each of the memory banks 20, 22 is a respective row address latch26, which stores the row address, and a row decoder 28, which appliesvarious signals to its respective memory bank 20 or 22 as a function ofthe stored row address. The row address multiplexer 18 also couples rowaddresses to the row address latches 26 for the purpose of refreshingthe memory cells in the memory banks 20, 22. The row addresses aregenerated for refresh purposes by a refresh counter 30, which iscontrolled by a refresh controller 32.

After the row address has been applied to the address register 12 andstored in one of the row address latches 26, a column address is appliedto the address register 12. The address register 12 couples the columnaddress to a column address latch 40. Depending on the operating mode ofthe SDRAM 10, the column address is either coupled through a burstcounter 42 to a column address buffer 44, or to the burst counter 42,which applies a sequence of column addresses to the column addressbuffer 44 starting at the column address that is stored in the columnaddress latch. In either case, the column address buffer 44 applies acolumn address to a column decoder 48, which applies various columnsignals to respective sense amplifiers and associated column circuitry50, 52 for the respective memory banks 20, 22.

Data to be read from one of the memory banks 20, 22 are coupled to thecolumn circuitry 50, 52 for one of the memory banks 20, 22,respectively. The data are then coupled to a data output register 56,which applies the data to a data bus 58. Data to be written to one ofthe memory banks 20, 22 are coupled from the data bus 58 through a datainput register 60 to the column circuitry 50, 52 and then aretransferred to one of the memory banks 20, 22, respectively. A maskregister 64 may be used to selectively alter the flow of data into andout of the column circuitry 50, 52, such as by selectively masking datato be read from the memory banks 20, 22.

The above-described operation of the SDRAM 10 is controlled by a commanddecoder 68 responsive to high level command signals received on acontrol bus 70. These high level command signals, which are typicallygenerated by a memory controller (not shown in FIG. 1), are a clockenable signal CKE*, a clock signal CLK, a chip select signal CS*, awrite enable signal WE*, a column address strobe signal CAS*, and a rowaddress strobe signal RAS*, with the “*” designating the signal asactive low or complement. The command decoder 68 generates a sequence ofcontrol signals responsive to the high level command signals to carryout the function (e.g., a read or a write) designated by each of thehigh level command signals. These control signals, and the manner inwhich they accomplish their respective functions, are conventional.Therefore, in the interest of brevity, a further explanation of thesecontrol signals will be omitted.

It will be appreciated that while this discussion mentions only twomemory banks 20, 22, four, eight etc. memory banks 20, 22 may be coupledtogether to provide a greater degree of data compression. It will alsobe appreciated that, while this discussion is in terms of compressingdata from multiple memory banks, data from multiple columns (or rows)within one memory bank may be compressed in a similar manner. In oneembodiment, the memory cells providing the data that are compressed arealso memory cells that would be replaced as a group by rows or columnsof redundant or spare memory cells (not illustrated). In thisembodiment, there is no need to test the memory banks 20, 22individually because, when a memory cell at one location is replacedwith a first redundant memory cell, a corresponding memory cell inanother location will be automatically replaced by a second redundantmemory cell.

In another embodiment, when testing determines from the compressed datathat a memory cell in at least one of the memory banks 20, 22 isdefective, individual testing of the memory cells in each memory bank20, 22 allows more precise identification of the defective memory cellwithout substantial compromise of testing speed.

FIG. 2 is a simplified schematic diagram of a DC sense amplifier 80 thatcan be used with an embodiment of the present invention. The DC senseamplifier 80 is of a type described in “A 40-ns 64-Mb DRAM with 64-bParallel Data Bus Architecture” by M. Taguchi et al., IEEE Journal ofSolid State Circuits, Vol. 26, No. 11, November 1991 and includes inputs82, 84′ coupled to PMOS transistors 90 and 92, each having a source andgate coupled to a respective one of the inputs 82, 84. The transistor 90is coupled to transistors 94, 96 in a current mirror configuration thatprovides a first current gain α₁ for a signal current I₁ on the input82. The transistor 92 is coupled to transistors 98, 100 in a currentmirror configuration also providing the first current gain α₁ for asignal current I₂ on the input 84.

NMOS transistors 102 and 104 form another current mirror amplifierhaving a current gain α₂. An output current I_(OUT) on the output 82′ isthus I_(OUT)=α₁I₁−α₁α₂I₂, and an output current I_(OUT)* on the output84′ is described similarly, but with I₁ and I₂ interchanged. This typeof DC sensing amplifier 80 provides high current sensitivity, which isadvantageous when the added capacitance of additional digit lines andI/O lines is coupled to the inputs 82, 84. This type of direct currentsense amplifier 80 also biases the inputs 82, 84 towards Vcc. As aresult, a wired-OR function may be realized by simply coupling two ormore open-drain FETs to one of the inputs 82, 84 to the DC senseamplifier 80.

FIG. 3 is a simplified schematic diagram of a portion of a memory device10 in accordance with an embodiment of the present invention. Inoperation, the column decoder 48 selects a column by activating either aread line 120 or a write line 122. The read line 120 is coupled to gatesof isolation transistors 124, 124′. The isolation transistors 124, 124′have sources coupled to ground and each has a drain coupled to a sourceof one of digit line read transistors 126, 126′. The digit line readtransistors 126, 126′ include gates coupled to digit lines D+ and D,respectively. As a result, a sense amplifier 130 (FIG. 4) can read datafrom a memory cell (not shown) in the memory bank 20 through the digitlines D, D+, and couple the data from the memory cell to the data bus58. Similarly, data from the data bus S8 can be written to the memorycell when the column decoder 48 activates the write line 122 to turn onthe write isolation transistors 136, 136′.

Because the digit line read transistors 126, 126′ have drains coupled tothe data bus 58, and the DC current sensing amplifier 80 of FIG. 2supplies a pull-up bias at the inputs 82, 84, which are also coupled tothe data bus S8, drains of multiple digit line read transistors 126,126′ corresponding to different memory cells can be coupled together toone of the inputs 82, 84 to provide a wired-OR function. As a result,data from multiple memory cells may be compressed, allowing multiplememory cells to be read simultaneously.

FIG. 4 is a block diagram of a portion of the memory device 10 of FIG. 1including a test circuit 140 in accordance with an embodiment of thepresent invention. To understand the inventive test circuit 140, it isbest to have an understanding of a portion of the circuitryconventionally used in the column circuitry 50, 52 in the SDRAM 10 ofFIG. 1. With reference to FIG. 4, the column circuitry 50, 52 for eachof the memory banks 20, 22 typically includes one of the senseamplifiers 130, 130′ for each column in each memory bank 20, 22,respectively. The sense amplifiers 30, 130′ each receive signals from,and apply signals to, a pair of complementary digit lines 142, 144 and146, 148, respectively. Thus, for example, when data being read from acolumn in memory bank 20 is “1,” the digit line 142 will be at a logic“1” and the complementary digit line 144 will be at a logic “0.”

As mentioned above, many sense amplifiers 130, 130′ are provided foreach array 20, 22, respectively. In one embodiment, the digit lines 142,144 and 146, 148 of each sense amplifier 130, 130′ are selectivelyapplied to complementary I/O lines 150, 152 and 154, 156 by columnaddressing circuitry 160. There is one pair of I/O lines 150, 152 and154, 156 for each segment of the arrays 20, 22, respectively. Inoperation, when the column for which the sense amplifier 130 is providedis addressed, the digit lines 142, 144 are coupled to the I/O lines 150,152, respectively, by the column addressing circuitry in the columncircuitry 50 (see FIGS. 1 and 3). Similarly, when the column to whichthe sense amplifier 130′ is coupled is addressed, the digit lines 146,148 are coupled to the complementary I/O lines 154, 156, respectively,by the column addressing circuitry in the column circuitry 52 (FIG. 1).

In response to signals from a test mode controller 159, the columnselect circuitry 160 selectively couples the I/O lines 150-156 to a pairof complementary data lines 162, 164 that in turn are coupled to inputsof a NOR gate 170. The column select circuitry 160 is formed bymultiplexers 172, 174, 176 and 178 and inverters 180 and 182. The senseamplifiers 130, 130′ of FIGS. 1 and 4 have previously been turned on.Each line of each pair of I/O lines 150-156 is thus at a voltage ofeither Vcc or ground and is coupled to an input of multiplexers 172-178.

In another embodiment, only the columns of interest are turned on. As aresult, the column addressing circuitry 160 is not required, and I/Olines 150 and 154 are both coupled to the data line 162. Similarly, theI/O lines 152 and 156 are both coupled to the data line 164.

As a result, in either embodiment, a wired-OR function is realized when,e.g., I/O lines 150, 154 are coupled to a common node that includes apull-up current source, such as the inputs 82, 84 to the DC senseamplifier 80 of FIG. 2. Thus, when the data being read out from thememory banks 20, 22 differ, both of the data lines 162, 164 will be atlogic “0.” The data lines 162, 164 are, in turn, coupled to the inputs82, 84 of the DC sense amplifier 80 having complementary outputterminals 82′, 84′ that are normally coupled to or part of the dataoutput register 56 (FIG. 1). When both of the data lines 162, 164 are atlogic “0,” this is detected by the NOR gate 170, and a logic “1” on theline FAIL coupled to an output of the NOR gate 170 indicates that one ofthe memory banks 20, 22 includes a defective memory cell.

In a normal mode of operation, the column decoder 48 selects only onecolumn at a time. In a test mode of operation in accordance with anembodiment of the invention, multiple columns are active at the sametime. In one embodiment, a wired-OR function compresses data from bothmemory banks 20, 22 by coupling both sets of I/O lines 150, 152 and 154,156 to the DC sense amplifier 80. When the same data have been writtento both memory banks 20, 22, but the read data from the two memory banks20, 22 are different, both data lines 162, 164 will go to logic “0.”

It will be appreciated that combinatorial logic may be used to detectfailed memory cells, even in memories having very wide data paths thatmay be coupled to other circuitry in a manner different that that shownin FIG. 4, e.g., not necessarily having I/O lines and data lines.

In one embodiment, weak latches 190 are coupled to each of the datalines 162, 164. A weak latch 190 is provided by coupling the input of aninverter 192 to one of the data lines 162, 164 and the output of thatinverter 192 to the gate of a PMOS transistor 194 having a sourcecoupled to ground and a drain coupled to that data line 162, 164. Theweak latches 190 may be set or reset in response to changes in the datapresented on the data lines 162, 164. The PMOS transistor 194 allows thelatch 190 to be set or reset by signal sources having more robust signalassertion capabilities.

In operation, the SDRAM 10 is tested by first writing known data to thememory banks 20, 22 by conventional means. The data may be written byaddressing individual memory cells or by using conventional approachesfor writing data to the memory banks 20, 22 for test purposes, asexplained above. For example, a logic “1” may be written to each memorycell in the memory banks 20, 22 so that when the data are read, thenon-complementary digit lines 144, 146 will be logic “1” and thecomplementary digit lines 142, 148 will be logic “0.” Under thesecircumstances, the DC sense amplifier 80 will receive the same signalsas when reading a logic “1” from a memory cell (i.e., D=“1” and D*=“0”)thereby generating a logic “1” on its output terminal 82′ and a logic“0” on its output terminal 84′. However, it will be understood thatlogic “0” may be written to all memory cells in the memory banks 20, 22,or data in some other pattern, such as a checkerboard pattern, may bewritten to the memory banks 20, 22. In any case, as long as the data arewritten to the memory banks 20, 22 in a known pattern, the compresseddata applied to the DC sense amplifier 80 and the NOR gate 170 canprovide an indication of whether the memory cells in the memory banks20, 22 are operating properly. However, the data written to the memorybanks 20, 22 must be selected so that, when the data is read from thememory banks 20, 22, the data and data* will be at complementary logiclevels.

A testing system 300 for testing the SDRAM 10 containing the testcircuit 140 of FIG. 4 is shown in FIG. 5, in accordance with anembodiment of the present invention. The SDRAM 10 is connected throughits address bus 14, data bus 58 and control bus 70 to a tester 306. Thetester 306 includes an addressing circuit 312 connected to the addressbus 14, a data generator 314 connected to the data bus 58, a dataanalyzer 316 also connected to the data bus 58, a mode controller 318connected to a plurality of externally accessible terminals of the SDRAM10 in one or more of the buses 14, 58, 70, and a memory controller 320providing control signals to the SDRAM 10 on the control bus 70.

In operation, the data generator 314 applies predetermined data to thedata bus 58 while the addressing circuit 312 applies suitable addressesto the address bus 14 to cause the data on the bus 58 to be written intothe SDRAM 10 under control of the memory controller 320. During thistime, the mode controller 318 generates and applies combinations ofsignals to the external terminals of the SDRAM 10 to cause the test modecontroller 132 to generate the bank addressing signals as desired. Afterdata have been written to the memory banks 20, 22, the mode controller318 generates a combination of signals to cause the test mode controller159 (FIG. 4) to place the test circuitry 140 in the test mode. Theaddressing circuitry 312 then addresses the memory cells of the memorybanks 20, 22 under control of the memory controller 320 to applycompressed data to the DC sense amplifier 80. The resulting data signalsat the output terminals 82′, 84′ of the DC sense amplifier 80 arecoupled through the data bus 58 to the data analyzer 316. The dataanalyzer 316 determines whether the data read from the SDRAM 10corresponds to the data written to the SDRAM 10. Although the testsystem 300 is shown as being used to test an SDRAM 10, it will beunderstood that it may be used to test other types of DRAMs and othertypes of memory devices.

The addressing circuits 312, data generator 314, data analyzer 316, modecontroller 318, and memory controller 320 can be implemented by avariety of means with relative ease by one skilled in the art. Forexample, these components can be implemented in software executed by acomputer system. Alternatively, the addressing circuit 312 can beimplemented by a counter that outputs incrementally increasingaddresses. The data Generator 314 can be implemented by a set of pull-upresistors that simply hold the lines of the data bus 58 at logic “1.”The data analyzer 316 can be implemented by a latch that detects apredetermined logic level. The mode controller 318 can be implemented bya logic circuit that generates predetermined combinations of signals.The memory controller 320 can be implemented by a conventional memorycontroller. The tester 306 and its internal components are preferablycontrolled by a conventional computer system (not shown).

FIG. 6 is a block diagram of a computer system 400 which includes theSDRAM 10 of FIGS. 1, 3 and 4. The computer system 400 includes aprocessor 402 for performing various computing functions, such asexecuting specific software to perform specific calculations or tasks.The processor 402 includes a processor bus 404 that normally includesthe address bus 14, the data bus 8, and the control bus. In addition,the computer system 400 includes one or more input devices 414, such asa keyboard or a mouse, coupled to the processor 402 to allow an operatorto interface with the computer system 400. Typically, the computersystem 400 also includes one or more output devices 416 coupled to theprocessor 402, such output devices typically being a printer or a videoterminal. One or more data storage devices 418 are also typicallycoupled to the processor 402 to allow the processor 402 to store data orretrieve data from internal or external storage media (not shown).Examples of typical storage devices 418 include hard and floppy disks,tape cassettes and compact disk read-only memories (CD-ROMs). Theprocessor 402 is also typically coupled to cache memory 426, which isusually static random access memory (“SRAM”) and to the SDRAM 10 througha memory controller 430. The memory controller 430 normally includes thecontrol bus 70 and the address bus 14 that is coupled to the SDRAM 10.The data bus 58 may be coupled to the processor bus 404 either directly(as shown), through the memory controller 430, or by some other means.

Although the present invention has been described with reference to apreferred embodiment, the invention is not limited to this preferredembodiment. Rather, the invention is limited only by the appendedclaims, which include within their scope all equivalent devices ormethods which operate according to the principles of the invention asdescribed.

1-64. (canceled)
 65. A method of simultaneously testing memory cells inat least two different columns of memory cells, the method comprising:coupling complementary bits of write data to respective complementarydigit lines for the at least two different columns of memory cells;coupling at least one digit line in each of the pairs of complementarydigit lines to a respective memory cell; decoupling the at least onedigit line in each of the pairs of complementary digit lines from therespective memory cell; subsequently coupling the at least one digitline in each of the pairs of complementary digit lines to the respectivememory cell; coupling the complementary digit lines for the at least twodifferent columns of memory cells to the respective complementary datalines; determining if voltages on the complementary data linescorrespond to complementary logic levels; and indicating a failcondition if the voltages on the complementary data lines do notcorrespond to complementary logic levels.
 66. The method of claim 65wherein the act of subsequently coupling the at least one digit line ineach of the pairs of complementary digit lines to the respective memorycell further comprises: coupling the digit lines in each of the pairs ofcomplementary digit lines to a respective sense amplifier; and allowingthe sense amplifier to drive the digit lines in each pair to voltagescorresponding to complementary logic levels.
 67. The method of claim 66wherein the act of allowing the sense amplifier to drive the digit linesin each pair to voltages corresponding to complementary logic levelscomprises allowing the sense amplifier to drive the digit lines in eachpair to voltages corresponding to complementary logic levels before theact of coupling the complementary digit lines for the at least twodifferent columns of memory cells to the respective complementary datalines.